1. Field of the Invention
This invention relates to a magnetic tape recording and/or reproducing apparatus, and more specifically, to a magnetic tape recording and/or reproducing apparatus having a capstan against which a pinch roller presses for feeding a magnetic tape at a constant speed from a supply reel past a recording and reproducing head, and wherein a predetermined small amount of the magnetic tape is returned past the head toward the supply reel when the tape is stopped.
2. Description of the Prior Art
Generally, it is preferable that a magnetic tape recorder which is stopped while recording be able to begin recording anew precisely at the end of the previously recorded material without erasing any of the previously recorded material. To accomplish this, a mechanism for returning a small amount of tape toward the supply reel at the interruption of a recording operation is generally provided in the magnetic recorder. This mechanism pulls back a small amount of tape from the recording or reproducing section, for example, from around the tape guide drum in a helical scan VTR, when the recorder is stopped.
A given tape amount returning method is conventionally used as a simple method for drawing a given amount of a magnetic tape, suitably 3 to 60 mm back from the head. According to this method, two or more posts that guide the magnetic tape are spring loaded and moveable. When the tape motion is stopped, tension of tape on the supply side of a guide drum is reduced while the tension on the take-up side is increased. The spring loaded guide post on the supply side therefore is permitted to move the tape against the reduced tension in that location to form a tape loop while the spring loaded guide post on the take-up side is forced by the increased tension to give up a tape loop it previously held. Thus, when the tape is stopped, the transfer of the tape from one tape loop to the other shifts the tape past the head toward the supply side.
The prior art is further described with reference to FIG. 1 which shows a conventional arrangement of the prior art wherein the given amount returning method is applied to a helical scanning type VTR. In the recording or play-back mode, a magnetic tape 2 from a supply reel 1 is guided by stationary posts 3 and 5 and moving post 4, then wound around a guide drum 6 which contains the recording and/or reproducing heads, and is held against a capstan 7 by means of a pinch roller 8 so as to be fed at a constant speed. The tape is further guided by stationary posts 9, 10, 11, 12, 13 and 15 and moving post 14, and is accumulated an a take-up reel 16. During recording or play-back, reel brakes 17 and 18 remain disengaged from reel hubs 19 and 20 as shown in FIG. 1. The moving post 4 is fixed to one end of an arm 21 which is pivotally mounted at the other end, and is urged by a spring 22 in the clockwise direction. When the magnetic tape 2 is being fed at a constant speed, the moving post 4 is in position A in balance between the tension of the tape and the force of the spring 22. In position A, moving post 4 forms virtually no tape loop.
Recording or reproduction is stopped by simultaneously applying the brakes 17 and 18 to the supply and to take up reel hubs 19 and 20 and disengaging the pinch roller 8 from the capstan 7. The moving post 14 moves from position B to position C so as to eliminate the loop of tape 2 in response to the increased tension in this part of the stopped tape. Moving post 4, being pulled by the spring 22, moves from position A to position D thus increasing the length of the loop in response to the reduced tension in that location. A given amount of the magnetic tape around the guide drum 6 which contains magnetic recording and reproducing heads is returned counterclockwise about the guide drum from the tape loop about moving post 14 toward the supply reel 1 and is taken up in the enlarged loop formed by stationary posts 3 and 5 and moving post 4.
According to this method, the angles of embrace of the magnetic tape 2 on the moving posts 4 and 14 are large, and the resulting large frictional load frequently prevents completely returning a given amount of the tape from the moving post 14 to the moving post 4. Since the tape tension decreases as the tape approaches the moving post 14, and since the balance between tape rigidity and tension determines the angle of embrace of the tape wound on moving posts 4 and 14, any environmentdependent change in the friction coefficient between the tape and stationary and moving posts causes a change in the tape tension which then results in a varying amount of the tape being returned to moving post 4.
A further cause of variability of tape returned by this method derives from the use of the brakes 17 and 18 for stopping the magnetic tape when the pinch roller 8 is disengaged from the capstan 7. Reel inertia and speed change as a reel becomes filled or emptied of tape. This change causes a change in the stopping positions of the two reels 1 and 16 as the tape is transferred from supply reel 1 to take-up reel 16. This results in substantial change of the amount of tape returned to moving post 4 upon disengagement of the pinch roller 8 from the capstan 7.